[s-cars] New 5 cylinder vs. AAN

[John Cody Forbes]

Yeah it's all down to piston acceleration and crank offsets. 

On a long stroke engine one of the main reasons you get more torque is the increased leverage on the crank shaft. Think of it like using a longer ratchet to tighten a bolt. You are able to generate about the same force yourself, but via the longer ratchet handle you apply more torque to the bolt. In a long stroke engine the explosion of the air/fuel mixture is going to generate basically the same force as it would in a shorter stroke engine, but the increased offset of the rod journal vs. the crank centerline applies more torque.

Sounds great right? There's a caveat. At a given RPM figure any piston in any engine has to travel the distance of the stroke in the exact same amount of time. On a long stroke engine what this means is that the piston must travel faster to cover the greater distance in the same time. Remember that on every stroke the piston must accelerate from a stop, reach a maximum speed, then stop again and get forced in the other direction. If you speed up it's velocity you are increasing the amount of inertia and at some point the inertia becomes too great and, for instance, the connecting rod breaks in half when trying to stop the piston from meeting the cylinder head. More commonly the force of the rod trying to stop the piston becomes great enough to force the oil film between the crank and rod bearing out and the rod bearing contacts the crankshaft resulting in wear. In the real world the maximum average speed of a piston is about 25 meters per second. This means a theoretical engine with a stroke of 100mm must not exceed about 7500rpm. If the stroke were 75mm the engine could do about 10,000rpm before piston speeds become too great. Of course many other factors are involved (weight of the piston, weight of the connecting rod, and the ability to flow enough air to make it worthwhile), but that's the gist of it.

-Cody (mobile)

On Oct 14, 2010, at 11:29 PM, "JC" <jc at j2c3.com> wrote:

> By Jove I think he's got it! (nohomo)
> 
> Think Harleys and Muricun V8's.
> 
>> 
>> And on that note,
>> 
>> One would expect a higher torque generated from a longer
>> stroke cylinder.
>> Correct ?
>> On 2010-10-14, at 7:19 PM, JC wrote:
> 
> 
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